If a person born with music creates Christian music, should…

If a person born with music creates Christian music, should they be allowed to create music about other things? I honestly feel that other Christians are very hard on musicians. They themselves enjoy all sorts of artist and songs having nothing to do with darkness or sin. Artist and songs that are not Christian but that are good to dance to, love to and just hang out and listen to.
Why is it however, that when an artist makes Christian music, if they do any sort of art outside of it afterwards they are considered to be sold out? Or even worse evil! When artist who paint do Christian art work or paintings paint flowers or faces of children, moon lit skies no one calls them sell outs or evil for it.

Can someone answer, why is it wrong for Christian music artist to create music about other things?